An ultrasonic sensor for use in a vehicle is generally mounted to a front/rear bumper of the vehicle. For example, an ultrasonic sensor disclosed in JP-2004-264264A is mounted to an inner wall of the bumper in such a manner that a vibrating surface of the ultrasonic sensor is exposed to an outside environment through a hole that penetrates through the bumper. The hole acts as a mounting location mark that indicates a mounting location of the ultrasonic sensor. Thus, the ultrasonic sensor is mounted in a correct location on the bumper. However, the bumper has a bad look due to the hole. Further, since the ultrasonic sensor is partly exposed to the outside environment through the hole, the ultrasonic sensor is easily damaged by gravel, water, and the like. Therefore, the ultrasonic sensor may have less reliability and a short life.
An ultrasonic sensor disclosed in WO9502237A1, corresponding to JP-3469243, is mounted to the inner wall of the bumper and uses a portion of the bumper as a vibrating surface. Thus, the ultrasonic sensor is invisible from the outer side of the bumper so that the bumper has a good look. However, since the bumper has no hole acting as the mounting location mark, the ultrasonic sensor may be mounted in an incorrect location on the bumper. This may result in a detection area variation, a sensor characteristic variation, a directivity variation, and a fixation reliability reduction. If the mounting location mark is formed on the inner wall of the bumper, the ultrasonic sensor may be mounted in the correct location on the bumper by using the mounting location mark as a reference. However, since the ultrasonic sensor uses the portion of the bumper as the vibrating surface, the mounting location mark may affect the sensor characteristic.